Supervision and monitoring of signals, such as clock signals and data signals, is of vital importance and signal monitors of various types can be found in many digital electronic systems such as telecommunication systems and other digital communication systems. For example, interfaces and clock paths for a synchronous digital system must continuously be supervised in order to maintain high reliability.
Basically, a signal monitor is an arrangement for monitoring the behavior and/or status of a signal. As long as the signal behaves as normal, the signal monitor confirms the existence of a valid signal. On the other hand, if the signal behavior deviates from normal, an alarm is normally raised so that appropriate actions can be taken.
The performance evaluation of a signal monitor is normally based on detection time and reliability. In high-performance applications, it is extremely important that the monitor has a fast detection speed so that a signal failure can be detected as soon as possible. High reliability is also essential and it is normally required that the monitor can handle different metastability situations.
Conventional signal monitors are often specialized application-dependent circuit solutions that require an intimate knowledge of the particular systems in which the monitors are applied. This generally means that the signal monitors are customized for signals with certain characteristics, thus making it difficult to adapt the signal monitors to system or application changes that imply supervision of signals with other characteristics.
General detection mechanisms that are capable of handling different types of signals under various conditions are normally very complex and require extensive data processing. Existing general detection mechanisms have worked quite well for monitoring of signals of low and moderate frequency. In high-frequency applications, however, the detection time latency is generally far too high to be acceptable. In many applications, the time from the actual signal failure until the alarm is activated may be several clock cycles. For high-frequency signals, there is simply not enough time for a complex detection mechanism to detect a signal failure in just a few clock cycles. Naturally, it would be beneficial if the reaction time was less than one cycle and close to ideal pulse width detection, even for high-performance and high-frequency applications.
In high-frequency applications, the monitored signal generally has to be sampled by a sample clock of much higher frequency, so-called oversampling. However, when the frequency of the monitored signal reaches higher and higher levels, it may not be possible to generate a high-frequency sample clock for oversampling purposes, at least not to a reasonable cost.
Therefore, most general loss detectors or signal monitors of today operate in a fairly moderate frequency range where oversampling and complex decision state machines are still options.